Determination of the position of a radiographic or radioscopic unit

ABSTRACT

The invention relates to a method for the determination of the position of a radiographic or radioscopic unit ( 16 ) with relation to a reference point (R ref ) on producing a radiographic image of an object ( 10 ). The position of the unit ( 16 ) with relation to a reference point (R ref ) is determined from the determination of the position of a test pattern ( 25 ), with relation to the unit ( 16 ), which is mechanically fixed to the object, by using the image of the test pattern with relation to the reference point (R ref ). The invention further relates to a determining device for carrying out said method.

CLAIM FOR PRIORITY

This application is the national stage application under 35 U.S.C. §371of the International Application No. PCT/FR2003/01917, and claims thebenefit of French Application No. 02/01726, filed Jun. 20, 2002 andInt'l. Application No. PCT/FR03/01917, filed Jun. 20, 2003, the entiredisclosures of which are incorporated herein by reference in theirentireties.

The present invention relates to a method and a device for detecting theposition in space of a device providing images by means of X rays, forexample, a radiography or radioscopy device. Hereafter, any radiographyor radioscopy device comprising an X-ray source, an X-ray photographicplate type sensor, and a film digitization system, or a radiography orradioscopy system formed of an X-ray source and of a digital sensor, orany operating suite device of mobile C-shaped arm type, comprised of asource and of an analog or digital sensor, the source and the sensorbeing rigidly interconnected, will be designated as a radiographicdevice.

For certain surgical operations, the surgeon may previously havethree-dimensional images of the patient's region to be operated,obtained for example by means of a scanner. The images enable thesurgeon to prepare the operation. During the operation, the surgeon mayneed to know the exact position of a bone with respect to the tools orof a pin that he is inserting. It is then no longer possible to performa scanning to know such positions.

Radiographies under several angles of the patient's operated region arethen usually acquired by means, for example, of a radiographic device.The radiographies are then digitized and the different elements presenton the images are identified. By means of calculation algorithms, theelements identified on the radiographies can then be associated with thecorresponding elements of the scanner views and the respectivepositions, for example, of a pin and of tools can be determined. Thedetermination of the position of pins or tools with respect to theconsidered anatomic part or the anatomic region is performed by using atracking system to which is associated a reference frame called R_(ref).The tracking system may be based on an optical technology (such assystem POLARIS of NDI Company, Toronto, Canada), based on a magnetictechnology (such as system Fastrack of Polhemus Inc., USA), based on anultrasound technology (product of Zebris Company, Germany). The trackingsystem locates in reference frame R_(ref) the position of a rigidlocalization body by providing a transformation matrix between thereference frame associated with the rigid localization body and thereference frame associated with the tracking system. The rigidlocalization bodies may be formed of either emitter or receiver, orreflector, or re-emitter elements, according to the technology used.

When a rigid localization body is for example fixed to a vertebra and arigid reference body is fixed to a surgical instrument, it is possibleto control the position of the surgical instrument, the trajectoryand/or the position of the instrument being determined due to theradiographic images provided a radiographic device. To be able to usethe information contained in the radiographic images, it is necessary todetermine the parameters of the radiographic device and to link thereference frame associated with the radiographic device and thereference frame of the localization system.

The obtaining of radiographies and their use require a previous step ofdetermination of the operating parameters of the radiographic device,which is formed of a source projecting X-rays on an exposition surface,the object to be X-rayed being interposed between the source and theexposition surface. In more detailed fashion, the case in point is tomodel the projection performed by the radiographic device. A projectionis the operation which, to a point (x, y, z) in the three-dimensionalspace, associates a point (u, v) in the image obtained on the expositionsurface when an X-ray photograph is taken. The so-called back-projectionlines D(u, v) are determined, each line corresponding to all thethree-dimensional points in the space which project on pixel (u, v) ofthe image. The determination step enables obtaining the equations of theback-projection lines in reference frame R_(amp) associated with theradiographic device. This amounts to determining so-called intrinsicparameters which are inherent to the geometry, to the manufacturing ofthe radiographic device, and to the method of digitization of theobtained image, and determining the so-called extrinsic parameters,which correspond to the transformation between the reference frame ofradiography system R_(amp) and fixed reference frame R_(ref). If theradiographic device moves, there then remains to find the extrinsicparameters again. The algorithms for determining these parameters aredescribed in document “The calibration problem for stereo by O. D.Faugeras et al.” in Proc. Computer Vision and Pattern Recognition p.15-20, 1992).

The determination step may be obtained by using a determination target(calibration target) attached to the radiographic device. The knownmethods and devices disclose:

-   -   a target-holding robot in “Gestes medico-chirurgicaux assistés        par ordinateur”, S. Lavallee Ph.D. Thesis, 1989,    -   a target attached on the sensor described in document “Vissage        pédiculaire assisté par ordinateur”, P. Sautot, Ph.D. Thesis,        1994.

There also exist commercial systems (target directly attachable on thesensor, as for example, the product sold by Traxtal TechnologiesCompany, USA).

Once the determination step has been completed, and assuming that theequations of the back-projection lines are not modified with respect toreference frame R_(amp) in displacements of the radiographic device,radiographies of the tool are taken along different directions. If,however, the distortion problems due to the terrestrial magnetic fieldare not negligible, it is possible by a simple system to correct theequations of the back-projection lines (for example, by attaching a setof markers at the sensor periphery, which are visible on the image, theanalysis of the position variation on the image enables determining andquantifying the distortion variation).

The radiographies, the transformation between reference frame R_(amp)and reference frame R_(ref) being known, are used for the surgicalnavigation. One may for example:

-   -   project the position of a tool on the radiographic images;    -   readjust the radiographic images with images provided by an X        scanner; perform three-dimensional reconstructions by tomography        or by the use of deformable models (French patent no 9911848        filed on Sep. 17, 1999 by Laboratoire TIMC, entitled        “Reconstitution de surfaces en trois dimensions par utilisation        de modèles statistiques”—the inventors of which are Markus        Fleute, Stéphane Lavallée, and Laurent Desbat).

U.S. Pat. No. 5,769,861 filed by Brainlab Med Computersyst and entitled“Method and device for localizing an instrument” and U.S. Pat. No.6,370,224 of Sofamor Danek Group entitled “System and methods for thereduction and elimination of image artifacts in the calibration of X Rayimagers” can be mentioned. Product fluoronav™ of Sofamor Danek Company,USA, can also be mentioned. FIG. 1A schematically shows an object 10,for example, a patient's bone, to be X-rayed. Object 10 is motionlesswith respect to a fixed reference frame R_(ref). Object 10 is placedbetween a source 12, represented by a point, and an exposition surface13, represented by a fixed plane with respect to source 12, of aradiographic device 16. The radiography corresponds to image 14 obtainedon exposition surface 13. Radiographic device 16, that is, source 12 andexposition surface 13, is intended to be displaced with respect toobject 10 to take the different X-ray photographs.

It is desired to determine the equations of the back-projection lines(two lines 17, 18 being shown in FIG. 1) in reference frame R_(ref) atthe time when each photograph is taken to perform a subsequentprocessing of the obtained radiographies. It is thus necessary todetermine the geometric transformation enabling passing from referenceframe R_(amp) to reference frame R_(ref) for each shooting.

A rigid localization body 19 is assembled on radiographic device 16 tobe motionless with respect to reference frame R_(amp). A localizationsystem motionless in reference frame R_(ref) is capable of determiningthe position of rigid body 19 in reference frame R_(ref).

At the time when a radiography is performed, the position of mobile body19 with respect to reference frame R_(ref) is measured. It is thenpossible to determine the geometric transformation enabling passing fromreference frame R_(amp) to reference frame R_(ref) at the time when theX-ray photograph is taken. It is thus possible to determine theequations of the back-projection lines with respect to reference frameR_(ref) at the time when the photograph is taken.

Generally, two radiographies in different positions are acquired. Foreach radiography, the equations of the back-projection lines withrespect to reference frame R_(ref) are determined. By triangulation, itis then possible to determine a three-dimensional representation ofobject 10 that can be compared with preoperative images.

FIG. 1B shows elements of FIG. 1A. A rigid reference body 21, having itsposition determined by localization system 20, is attached on object 10to be X-rayed and enables detecting the possible displacements of theobject with respect to reference frame R_(ref). A rigid body 22 may beplaced on a surgical tool 23, having a mark on radiographic image 14that can be represented. The position of tool 23 can thus be determinedand used on analysis of the radiographic image. A target 24 (forexample, an anatomic point, or the point of insertion of a screw . . . )leaving a mark on the radiographic image can be aimed at on object 10 bytool 23.

The previously-described method for performing radiographies assumes aperfect synchronization between the obtaining of the radiography byradiographic device 16 and the measurement by localization system 20 ofthe position of rigid body 19 solid with radiographic device 16.

To ensure the synchronization, a system which detects the emission of Xrays by radiography tool 16 may be used at the time when a radiographyis performed, and then automatically controls the memorization bylocalization system 20 of the position of radiography tool 16 at thetime when the photograph is taken. The acquisition of the radiographyand the memorization of the position of radiography tool 16 may also becontrolled by a processor simultaneously connected to radiographicdevice 16 and to localization system 20.

However, such systems are difficult to form and require using expensivesensors or devices.

The present invention aims at providing a method and a device fordetecting the position of a radiographic device where an X-rayphotograph is taken which does not exhibit the abovementioneddisadvantages.

To achieve this object, the present invention provides a method fordetermining the position of a device providing images by means of X rayswith respect to a reference frame as an image of an object is taken, inwhich the position of the device with respect to the reference frame isdetermined based on the determination of the position with respect tothe device of a target, mechanically connected to the object, by meansof the impression of the target on the image, and on the determinationof the position of the target with respect to the reference frame.

According to an embodiment of the present invention, the position of thetarget with respect to the reference frame is determined from thedetermination, by a localization system, of the position with respect tothe reference frame of a rigid localization body mechanically connectedto the target.

According to an embodiment of the present invention, the target is fixedwith respect to the rigid body.

According to an embodiment of the present invention, the configurationof the target is determined by a feeler connected to a rigidlocalization body having its position with respect to the referenceframe determined by a localization system.

According to an embodiment of the present invention, the target isconnected to the rigid body by an articulated arm.

According to an embodiment of the present invention, the target isremoved from the object between the acquisition of two images.

According to an embodiment of the present invention, the determinationof the position of the target with respect to the device is performedfrom the determination on the image of characteristic impressions, eachcharacteristic impression corresponding to the projection on the imageof a separate element of the target.

The present invention also provides a target comprising elementstransparent to X rays and elements opaque to X rays, comprising at leastthree supports transparent to X rays, each support containing ballsopaque to X rays substantially aligned along a determined direction, thedetermined directions being non coplanar.

According to an embodiment of the present invention, at least two ballsare of different diameters.

According to an embodiment of the present invention, the targetcomprises a holding means capable of maintaining the cylinders accordingto a configuration from among several determined configurations.

The present invention also provides a device for determining theposition of a device providing images by means of X rays with respect toa reference frame when a radiography of an object is acquired,comprising a target connected to the object and comprising elementsopaque to X rays, each opaque element being capable of providing acharacteristic impression on the radiography of the object; a means fordetermining the position of the target with respect to the referenceframe; and a means for determining the position of the target withrespect to the radiographic device based on the characteristicimpressions of the radiography.

The foregoing object, features, and advantages, as well as others of thepresent invention will be discussed in detail in the followingnon-limiting description of specific embodiments in connection with theaccompanying drawings, among which:

FIG. 1A, previously described, illustrates in simplified fashion aconventional method for performing radiographies;

FIG. 1B, previously described, illustrates the use of radiographicimages to localize the position of a tool with respect to a determinedtarget;

FIG. 2 schematically illustrates a method for performing radiographiesaccording to the present invention;

FIG. 3 shows an example of the forming of a target according to thepresent invention;

FIG. 4 shows a variation of the target of FIG. 3;

FIG. 5 schematically shows an example of a radiography of the target ofFIG. 3;

FIGS. 6 and 7 shows two variations of the target of FIG. 3;

FIGS. 8 and 9 show two views of another variation of the targetaccording to the present invention;

FIGS. 10 and 11 show examples of determination of the targetconfiguration;

FIG. 12 schematically shows another variation of the target according tothe present invention; and

FIG. 13 shows an example of a radiography of the target of FIG. 12.

The present invention provides a device, the use of which enablesdetermining the geometric transformation enabling passing from referenceframe R_(amp) to initial reference frame R_(ref) at the time when aradiography is taken by directly using the image of an element of thedevice obtained on the radiography. The intrinsic parameters of theradiographic device may be previously determined according to aconventional procedure such as described in the document entitled“Vissage pédiculaire assisté par ordinateur” by P. Sautot, Ph. D.Thesis, 1994. The present invention provides a device, the use of whichsimplifies existing methods for determining the extrinsic parameterswhen the radiographic device has been displaced.

FIG. 2 shows source 12 of the radiographic device capable of emitting Xrays crossing object 10 to be X-rayed to form an image 14 on expositionsurface 13. A target 25 and a rigid body 26 are assembled on object 10to be X-rayed. Rigid body 26 is fixed with respect to a reference frameR_(mir) associated with target 25.

The method for acquiring radiographies of object 10 according to thepresent invention is the following. Prior to the radiographies, theequations of the back-projection lines, expressed in reference frameR_(amp), are determined. The position of rigid body 26 with respect toinitial reference frame R_(ref) is also determined. The relativepositions between rigid body 26 and target 25 being known, it ispossible to determine the geometric transformation T1 enabling passingfrom reference frame R_(mir) associated with target 25 to initialreference frame R_(ref). After, between several shootings, target 25 andobject 10 are superposed fixedly with respect to initial reference frameR_(ref), and geometric transformation T1 remains constant. If such wasnot the case, T1 should be determined again.

Radiographic device 16 is then displaced to different positions toperform radiographies. For each radiography, the analysis of theobtained image 14 enables, due to the presence of target 25 on object 10and as will be explained hereafter, determining the geometrictransformation T2 enabling passing from reference frame R_(amp)associated with radiographic device 16 to reference frame R_(mir)associated with target 25.

It is thus possible to determine the general geometric transformation TGenabling passing from reference frame R_(amp) associated with theradiographic device 16 to initial reference frame R_(ref). The equationsof the back-projection lines in initial reference frame R_(ref) can thenbe determined, and may be used to define a three-dimensional image ofobject 10 as explained previously.

FIG. 3 more specifically shows an example of a form of target 25. A base28 is temporarily attached to object 10 to be X-rayed, for example, apatient's femur. Base 28 supports rigid reference body 21 whichcomprises, for example, elements 29 capable of reflecting the infraredrays emitted by localization system 20. A flexible shaft 30 connectsbase 28 to a fork 32 which supports mobile body 26 at the end of abranch and target 25 at the end of the other branch.

It should be noted that target 25 and attached rigid body 26 could alsobe, for example, simply placed on the object 10, or placed on theoperating table, or clipped to a surgical drape, etc. . . . , providedthat target 25 is in the field of view of the X-ray image and rigid body26 is in the field of view of the tracking system 20 at the instant theX-ray image is acquired.

Target 25 is formed of several cylinders, 34A to 34E, connected at theirends by junction spheres 36. Each cylinder 34A to 34E is formed of amaterial transparent to X rays, for example a plastic matter or amaterial known under trade name Plexiglas, and comprises balls opaque toX rays 38, for example made of tungsten carbide, of lead, or of steel,substantially aligned along the axis of cylinder 34A to 34E.

Target 25 is formed of a central cylinder 34A, comprising for examplefive balls 38 of a first diameter, for example, six millimeters and, forexample, five balls 38 of a second diameter, for example, threemillimeters, smaller than the first diameter, the balls of the firstdiameter being located on one half of central cylinder 34A and the ballsof the second diameter being located on the second half. Two secondarycylinders 34B, 34C, comprising balls 38 of the second diameter, formwith central cylinder 34A a first triangle. Two secondary cylinders 34D,34E, comprising balls 38 of the first diameter, form with centralsegment 40 a second triangle. The planes containing the two trianglesare inclined with respect to each other.

FIG. 4 shows an alternative of the device of FIG. 3. Fork 32 is directlyassembled on object 10 to be X-rayed by an attachment means 40.According to this alternative, target 25 being fixed with respect toobject 10, a single rigid body 26 may be used directly to define theposition of target 25 and of object 10 with respect to initial referenceframe R_(ref) and the geometric transformation T2 enabling passing fromreference frame R_(mir) to initial reference frame R_(ref).

FIG. 5 shows an example of a radiography obtained from mobile target 25of FIG. 3 assembled on object 10 to be X-rayed. Balls 38 opaque to Xrays leave circular marks 42 on the radiography. Cylinders 34A to 34Btransparent to X rays leave substantially no mark on the radiography.Object 10 also leaves marks on the radiography which can superpose tothose of balls 38 and which are not shown in FIG. 5. The obtainedradiography is digitized. By using an appropriate algorithm, circularmarks 42 are determined. The positions of the centers of circular marks42, as well as segments of straight lines crossing said centers, arethen calculated. It is then determined to which cylinder 34A to 34E ofmobile target 25 does each segment of a straight line correspond, byespecially using the diameter difference of balls 42 and theirdistribution per cylinder 34A to 34E.

The equations of the back-projection lines of each pixel of theradiography are known in reference frame R_(amp) associated withradiographic device 16. Based on the back-projection lines, the shape oftarget 25 being perfectly well known, it is possible to determine theposition that target 25 must have in reference frame R_(amp) to generatemarks 42 obtained on the radiography. This may be obtained by theminimization of a quadratic criterion. The geometric transformation T2enabling passing from reference frame R_(amp) 25 associated with theradiographic device to reference frame R_(mir) associated with target 25is thus determined.

FIG. 6 shows an alternative forming of target 25 according to thepresent invention. Target 28 is formed of a junction sphere 44 assembledon a holding arm 46 attached on object 10 to be X-rayed. Cylinders 50A,50B, 50C are assembled on junction sphere 44 by attachment branches 52A,52B, 52C. Rigid body 26 is also assembled on junction sphere 44.Cylinders 50A, 50B, 50C are, as previously explained, formed of amaterial transparent to X rays and comprise balls 54 opaque to X rays.The different cylinders 50A, 50B, 50C must preferably be arranged not tobe coplanar. Junction sphere 44 comprises additional openings 56distributed across its entire surface possibly enabling addingadditional cylinders or arranging the three cylinders according to adifferent configuration.

FIG. 7 shows another alternative of target 25 according to the presentinvention. Two cylinders 58A, 58B are attached to the ends of the armsof a fork 60. A third cylinder 58C is attached to the base of fork 60.Target 25 is connected by a flexible rod 62 to a mount 64 attached toobject 10 to be X-rayed. Rigid reference body 21 is attached solid withmount 64. According to this alternative, there is no mobile bodyattached solid with target 25. In the determination of geometrictransformation T1 enabling passing from reference frame R_(mir) toinitial reference frame R_(ref), the determination of the position oftarget 25 with respect to rigid reference body 21 may be obtained bymeans of a feeler.

FIGS. 8 and 9 show two views of another variation of target 25. Object10 to be X-rayed is for example a vertebra. Target 25 comprises a fork66 having its arms 68A, 68B attached to object 10. Each arm 68A, 68Bsupports a cylinder 70A, 70B. A third cylinder 70C is attached to thebase of fork 66. Rigid body 26 is also attached to the base of fork 66.According to this alternative, target 25 being fixed with respect toobject 10, a single rigid body 26 may be used directly to define thepositions of target 25 and of object 10 with respect to initialreference frame R_(ref) and to determine the geometric transformation T1enabling passing from reference frame R_(mir) to reference frameR_(ref).

FIGS. 10 and 11 show examples of the determination of the targetconfiguration (a single cylinder 72 of the target being shown in FIGS.10 and 11) in the case where said mark is assembled on the object in anon-predefined manner, as can be the case with the examples of theforming of the target shown in FIGS. 3, 6, and 7. A feeler 74 connectedto a rigid body 76, having its position determined by localizationsystem 20, may be displaced on the target to determine its shape. Aspecific feeler of recessed-type 78 with a single position connected toa rigid localization body 80 and comprising a hollow cylinder 82 of adiameter corresponding to the diameter of target cylinders may also beused. Recessed system 78 is then adjusted on each of the targetcylinders.

FIG. 12 schematically shows another alternative of target 25 accordingto the present invention. According to such an alternative, target 25comprises ten opaque balls 54, nine balls having an identical diameterand one ball 84 having a greater diameter. Balls 54 are distributed intwo planes P1 and P2, five balls being associated with each plane. Theangle between planes P1 and P2 is strictly smaller than 180 degrees and,advantageously, on the order of 90 degrees. In plane P1, the balls aredistributed along two parallel axes A1, A2. Three balls are arrangedsubstantially equidistantly on axis A1 and two balls are arranged alongaxis A2 in quincunx with respect to the balls of axis A1. The largestball 84 is placed at the end of axis A1 most distant from plane P2. Inplane P2, the balls are distributed along two parallel axes A3, A4.Three balls are distributed equidistantly on axis A3 and two balls arearranged on axis A4 in quincunx with respect to the balls of axis A3.Axis A1 cuts axis A3 and axis A2 cuts axis A4. The plane defined by axesA1 and A3 forms with plane P1 an angle of approximately 90 degrees.

Target 25 must remain visible on exposition surface 13 whatever thedisplacement of source 12 and of exposition surface 13. In the casewhere source 12 and exposition surface 13 move on a sphere having itscenter corresponding to object 10, the source and the exposition surfacebeing diametrical, target 25 must be comprised within a sphere, centeredon object 10, having its radius r provided by the following relation:r=d*sin(α)/2 with tan(α)=l/2dwhere d is the distance between source 12 and exposition surface 13 andl is the width of exposition surface 13.

Axis A1 (respectively, A3) and axis A2 (respectively A4) are as spacedapart as possible, while respecting the previously-stated conditionconcerning the dimensions of target 25, to improve the numericalstability of the algorithm which determines the positions of thecircular marks corresponding to the images of balls 54 on expositionsurface 13.

FIG. 13 shows an example of a radiography obtained from mobile target 25of FIG. 12. The applicant has shown that with the structure of target25, the order of circular marks 42 along a direction u is identicalalmost independently from the positions of source 12 and of expositionsurface 13 with respect to target 25. The dimensions of target 25 beingsufficiently small with respect to distance d, circular mark 42associated with the largest ball 84 is greater than the circular marksassociated with the other balls. Thereby, by placing the largest ball 84so that the associated circular mark 42 is at the first position alongdirection u, the putting in correspondence of circular marks 42 andballs 54 is then trivial. Direction u for example corresponds to adirection characteristic of exposition surface 13 or to the main axis ofinertia of circular marks 42.

The use of ten balls contributes to the robustness of the circular markdetection algorithm. However, target 25 may comprise less than ten ballswithout excessively degrading the robustness of the detection algorithm.Two balls on axis A1 (including the largest ball), one ball on axis A2,two balls on axis A3, and one ball on axis A4, may for example beprovided.

The present invention has many advantages.

First, the present radiography method enables determining the geometrictransformation between reference frame R_(amp) associated with theradiographic device and initial reference frame R_(ref) when aradiography is performed, directly from the radiography by the analysisof the impression left on the radiography by a target attached to theobject to be X-rayed. It is then not necessary to permanently attach arigid body to the radiographic device, since the analysis of theimpression of the target on the radiography enables following the motionof the radiographic device and finding the rigid spatial transformationsbetween two different positions.

Second, the target may be formed in a light material to be able to beattached on the object to be X-rayed. In particular, the target does nothinder possible displacements of the object, in the case where forexample the object is a vertebra. For example, the target may weightless than 300 grams.

Third, the shape of the target may easily be adapted according to theobject to be X-rayed and/or to the surgical operation to be performed toavoid hindering the surgeon's gestures.

Of course, the present invention is likely to have various alterationsand modifications which will occur to those skilled in the art. Inparticular, the opaque balls may be distributed on element other thancylindrical. They may for example be transparent tubes describingthree-dimensional curves. Further, there may be more than two differentdiameters for the opaque balls. Finally, some of the features of thepreviously-described examples of embodiment may be combined.

1. A method for determining the position of a device providing images byX rays with respect to a reference frame as an image of an object istaken, said method comprising the steps of: determining the position ofa target with respect to the device, said target mechanically connectedto the object, based on an impression of the target on the image of theobject; determining the position of the target with respect to thereference frame; and determining the position of the device with respectto the reference frame based on the position of the target with respectto the device, and the position of the target with respect to thereference frame.
 2. The method of claim 1, in which the position of thetarget with respect to the reference frame is determined from thedetermination, by a localization system, of the position with respect tothe reference frame of a rigid localization body mechanically connectedto the target.
 3. The method of claim 2, in which the target is fixedwith respect to the rigid body.
 4. The method of claim 1, in which aconfiguration of the target is determined by a feeler connected to arigid localization body, the position of the feeler with respect to thereference frame being determined by a localization system.
 5. The methodof claim 2, in which the target is connected to the rigid body by anarticulated arm.
 6. The method of claim 1, in which the target isremoved from the object between an acquisition of a first image and anacquisition of a second image.
 7. The method of claim 1, in which thedetermination of the position of the target with respect to the deviceis performed from the determination on the image of the impression ofthe target, said impression comprising a plurality of characteristicimpressions, each of said plurality of characteristic impressionscorresponding to a projection on the image of a separate element of thetarget.
 8. The method of any of claims 1 to 7, wherein the targetcomprising: a plurality of elements transparent to X rays; a secondplurality of elements opaque to X rays; and wherein said first pluralityof elements comprises at least three supports transparent to X rays,each support containing said second plurality of elements comprising aplurality of balls opaque to X rays substantially aligned along adetermined direction, the determined directions being non coplanar. 9.The method of claim 8, in which at least two balls of said plurality ofballs are of different diameters.
 10. The method of claim 8, whereinsaid target further comprises a hold means capable of maintaining the atleast three supports according to a configuration from among severaldetermined configurations.
 11. A system for determining the position ofa device providing image by X rays with respect to a reference framewhen a radiography of an object is acquired, said system comprising: atarget connected to the object and comprising a plurality of elementsopaque to X rays, each of said plurality of opaque elements beingcapable of providing a characteristic impression on the radiography ofthe object; a means for determining the position of the target withrespect to the reference frame; a means for determining the position ofthe target with respect to the device based on the characteristicimpressions of the radiography, wherein said system is adapted todetermine the position of the device with respect to a reference framebased on the position of the target with respect to the reference frameon the position of the target with respect to the device.
 12. A methodfor determining the position of a device providing images by X rays withrespect to a reference frame as an image of an object is taken, saidmethod comprising the steps of: determining the position of a targetwith respect to said device based on an impression of said target on theimage of said object, said target being mechanically connected to theobject and including a plurality of elements opaque to X rays whichdefine the impression of the target; determining the position of saidtarget with respect to the reference frame using a rigid body that ismechanically connected to said target; and determining the position ofthe device with respect to the reference frame based on the position ofsaid target with respect to said device, and the position of said targetwith respect to the reference frame.
 13. The method of claim 12, whereinthe target includes a plurality of elements transparent to X rays thatincludes at least three supports transparent to X rays, each supportcontaining the opaque elements comprising a plurality of balls opaque toX rays, the balls being substantially aligned along a determineddirection, the determined directions being non coplanar.
 14. The methodof claim 13, in which at least two balls of said plurality of balls areof different diameters.
 15. The method of claim 13, wherein said targetfurther comprises a hold means capable of maintaining the at least threesupports according to a configuration from among several determinedconfigurations.